System and method for cutting using a variable astigmatic focal beam spot

ABSTRACT

A variable astigmatic focal beam spot is formed using lasers with an anamorphic beam delivery system. The variable astigmatic focal beam spot can be used for cutting applications, for example, to scribe semiconductor wafers such as light emitting diode (LED) wafers. The exemplary anamorphic beam delivery system comprises a series of optical components, which deliberately introduce astigmatism to produce focal points separated into two principal meridians, i.e. vertical and horizontal. The astigmatic focal points result in an asymmetric, yet sharply focused, beam spot that consists of sharpened leading and trailing edges. Adjusting the astigmatic focal points changes the aspect ratio of the compressed focal beam spot, allowing adjustment of energy density at the target without affecting laser output power. Scribing wafers with properly optimized energy and power density increases scribing speeds while minimizing excessive heating and collateral material damage.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of co-pending U.S.Provisional Patent Application Serial No. 60/448,503, filed on Feb. 19,2003, which is fully incorporated herein by reference.

TECHNICAL FIELD

[0002] This invention relates to laser cutting or machining, and moreparticularly, relates to a system and method for forming a variableastigmatic focal beam spot using solid-state lasers with an anamorphicbeam delivery system to scribe semiconductor wafers.

BACKGROUND INFORMATION

[0003] Increasing worldwide demand for compound semiconductor deviceshas driven the development of high throughput, cost effective, andreliable production techniques and equipment. Compound semiconductorsare comprised of a combination of group-III elements, such as B, Al, Ga,In and Ti, and group-V elements, such as N, P, As, Sb and Bi. A typicalexample of III-V compound semiconductor devices is light emitting diodes(LEDs) using InP, GaP, GaAs, AlInGaP and GaN.

[0004] Among theses LEDs, blue LEDs consist of multiple GalliumNitride-based layers epitaxially grown on a silicon carbide or sapphirewafer substrate. Silicon carbide wafers have been diced usinghigh-precision saws. Sapphire wafer die separation has been performed bymechanical scribing with a diamond tool. The wafer can then be cleavedalong the scribed lines via a fracturing machine. The extreme hardnessof blue LED substrates and the small LED die size create significantproblems for both saw dicing and mechanical scribing, including low dieyield, low throughput, and high operating costs. The brittle substrates,such as GaP and GaAs, also show low productivity due to excessive edgechipping by the mechanical scribe and break and the saw dicingprocesses. Moreover, the conventional processes require relatively largecutting areas, reducing the number of devices on a wafer.

[0005] Scribing with ultra violet (UV) lasers has emerged as analternative method for separating compound semiconductor wafers. Excimerlasers and diode pumped solid-state (DPSS) lasers are two major lightsources for UV laser scribing. When short duration UV laser light pulsesare tightly focused onto the wafer surface, each pulse is absorbed intoa sub-micron thick surface layer, which then vaporizes. The vaporizedmaterial carries away the energy of the interaction, minimizing heattransfer to the surrounding material. This process is known asphoto-ablation. In order to produce deep cuts, hundreds of successivelaser pulses are focused onto the surface.

[0006] Moving the wafer under a rapidly pulsed, focused laser beamproduces an extremely narrow ‘V’ shaped cut, the depth of which iscontrolled by the scan speed. Typically, these cuts terminate 30-50%into the thickness of the wafer. After laser scribing, the wafer isfractured using standard cleaving equipment. The ‘V’ shaped laser cutsact as stress concentrators, inducing well-controlled fracturing withexcellent die yield.

[0007] Efficient photo-ablation is preferred for laser scribing anddepends strongly on two properties of the UV laser light: wavelength andpulse duration. In general, photo-ablation benefits from shorter laserwavelength and shorter pulse duration for both optical and thermalreasons. In the formula, E=h(c/λ), the photon energy, E, is inverselyproportional to λ, the photon wavelength. Simply put, shorterwavelengths impart more energy per photon. The benefits achieved byshort laser wavelengths include improved optical absorption, reducedabsorption depth, lower irradiance required for ablation, and reducedcut width. In the formula, I=E/At, the irradiance, I, is proportional topulse energy, E, and inversely proportional to both beam area, A, andpulse duration, t. As a result, short pulse durations result in higherirradiances for a given spot size and pulse energy. Also, short pulsesdeliver successful ablation at larger spot sizes on target, resulting inmore rapid cutting. The benefits achieved by short laser pulse durationinclude increased irradiance on target, and reduced heat transfer to thesubstrate due to more rapid absorption and ablation.

[0008] In silicon carbide, optical wavelengths below 370 nm have photonenergies that exceed the bandgap of the material, resulting in directphoton absorption. For example, the photonic energy of a DPSS 355 nmsolid state laser beam (3.5 eV) is higher than the highest bandgap ofsilicon carbide (3.27 eV for 4H polytype), resulting in strongabsorption followed by ablation. Sapphire, conversely, has a bandgap(9.9 eV) that is higher than the photon energy of commercially-availableUV lasers, for example, an F₂ laser at 157 nm (7.9 eV). In such cases,multi-photon absorption can induce efficient optical absorption and thenecessary irradiance (W/cm²) for multi-photon absorption can be veryhigh. The efficiency of multi-photon absorption in sapphire is stronglywavelength-dependent. Shorter wavelengths are absorbed more completelyin sapphire, resulting in less heat input to the bulk material. Forphoto-ablation to occur, light that is absorbed must impart sufficientenergy to vaporize the material. The threshold irradiance for ablationis also strongly wavelength-dependent. The higher photon energy andsmaller absorption depth of shorter optical wavelengths result inablation at lower irradiance levels.

[0009] As compared to the sapphire, compound semiconductor substratesusually have lower bandgap energy, such as GaN (3.3 eV), GaP (2.26 eV)and GaAs (1.42 eV). Although coupling of a UV laser at 266 nm isefficient on these substrates, excessive photonic energy under thisstrong absorption can result in unnecessary thermal conduction to thesubstrates, causing heat related damages. In contrast, insufficientlaser energy density can result in improper ablation, even with thestrong absorption. Thus, an optimum laser energy density or irradianceis an important factor in laser scribing, which leads to a higherscribing speed and/or maximized productivity.

[0010] Among UV lasers, excimer lasers generate the most power output,e.g., nearly 100 watts in the deep UV range. These advantages have madethe excimer laser uniquely suited for hard LED wafer scribing. Scribingusing excimer laser technology has involved the delivery of a line beamor series of line beams onto an LED wafer, which is translated by acontrolled motion stage to be diced in a desired direction. Excimerlaser scribing, for example, using KrF 248 nm light output, utilizes anear-field imaging technique through which a patterned laser beam isprojected from a mask. Thus, the delivery of a line beam has beenachieved by line-patterned mask projection. One example of the use ofexcimer laser patterned projections is disclosed in U.S. Pat. No.6,413,839, incorporated herein by reference. In this patent, a singleline beam and multiple line beams are projected onto a sapphire waferwith blue LEDs.

[0011] When using a mask with excimer laser projection techniques, themodification of a patterned beam is relatively simple and is achieved bychanging the shape of the mask. For example, the narrow line beam forscribing (i.e., usually in tens of microns) is projected by a slit mask.However, the slit mask transmits the laser beam only through an openarea of the mask. Thus, introducing the mask in a beam delivery system(BDS) blocks a major portion of the laser beam, which makes the beamutilization factor (BUF) low. This low beam utilization factor limitsthe speed of the scribing process, because the scribing speed is mainlyproportional to the size of the projected beam in the scribingtranslation direction.

[0012] Recently, developments in UV solid-state laser technology haveresulted in DPSS lasers with sufficient average power to be consideredfor the scribing of hard compound semiconductor wafers, such as thosemade of sapphire and silicon carbide. A few laser manufacturers havedeveloped the third harmonic (355 nm) and the fourth harmonic (266 nm)DPSS lasers with a gain medium for Nd³⁺ ions doped in ayttrium\crystalline matrix (Nd:YVO₄ or Nd:YAG). Thesefrequency-multiplied DPSS lasers demonstrate significant improvements inpulse duration, frequency and power output. For example, UV solid-statelasers running at the third harmonic (355 nm) now achieve average powersof over 5 Watts, and the fourth harmonic (266 nm) lasers achieve averagepowers of over 2.5 Watts. Also, these lasers offer short pulsedurations, e.g., below 20 nanoseconds. UV solid-state lasers with shortwavelength and pulse duration (e.g., less than 15 nanoseconds) cancreate extremely high irradiance, e.g., over 10⁹ W/cm², resulting ininstantaneous vaporization by photonic bombardment. This extremeirradiance of frequency-multiplied DPSS lasers makes the separation ofthe hard substrate possible. For an example, although the sapphire has ahigh optical transmissivity to UV wavelengths, this extreme temporal andspatial concentration of photons results in effective multi-photonabsorption, causing ablation.

[0013] Generally, UV solid-state lasers generate a circular Gaussianbeam in TEM₀₀ mode and current methods of UV solid-state laser scribingutilize a focused circular beam spot. Unlike an excimer laser BDS, DPSSlasers utilize far field imaging, which does not require patterned maskimaging. Examples of laser scribing using far field imaging aredisclosed in U.S. Pat. No. 5,631,190 and U.S. Pat. No. 6,580,054,incorporated herein by reference. The raw beam from the laser resonatoris directly focused by a beam-focusing lens and is delivered to thetarget. The BUF is higher because the BDS for a DPSS laser utilizes thefull beam. The scribing speed is slower, however, due to the small sizeof the focused beam spot, which is one drawback of the application ofthe DPSS laser to mass-production. Also, the conventional beam deliverysystem used in a DPSS laser has a limited ability to control theadjustment of laser processing parameters. In the conventional scribingtechniques using DPSS lasers, laser processing parameters are controlledby adjusting laser output power and the directed laser light is notmodified.

[0014] In general, laser beams must be focused for laser materialprocessing applications. A focused laser beam has two importantcharacteristics; 1) optimum laser intensity (usually expressed by thelaser energy density J/cm²) for a target material, and 2) minimum sizeof a focused spot or a beam waist diameter. The optimum laser intensityis important to achieving a desired processing result, because eitherexcessive or insufficient laser intensity will introduce imperfectionsinto the process. In addition, the focused beam spot should havesufficient flexibility to adjust its intensity, since the optimumintensity is determined by the light absorption properties of theparticular target material. The minimum size of a beam waist diameter isimportant when laser material processing requires a sharply focused beamfor fine resolution.

[0015] Another issue with laser scribing processes is caused by theablation induced debris generated along the wake of the cut. The debrison the semiconductor dies or LED dies are detrimental to both theirperformance and packaging. Photoresists for lithography have beenapplied on substrate surfaces for the protective coating to prevent thedebris, but the photoresist tends to be carbonized by the heat from thelaser induced plasma. The carbonized photoresist is hard to remove,especially near the laser cuts. Adhesive tapes have also been proposedas protection, but related procedures, such as changing the adhesivetape after scribing in every single direction, are not conducive to massproduction. In addition, excessive amounts of residue remained after thelaser scribing because of the high thickness of the tape together withthe adhesive.

[0016] Accordingly, there is a need for a laser scribing system andmethod that avoids the drawbacks of the existing techniques, is capableof using shorter wavelengths and pulse duration, and is capable ofoptimizing laser intensity and minimizing beam waist diameter.

SUMMARY

[0017] In accordance with one aspect of the present invention, a methodis provided for forming a variable astigmatic focal beam spot to cut asubstrate. The method comprises the steps of generating a raw laserbeam, expanding the raw laser beam, and modifying the expanded beam suchthat the modified beam is collimated in one principal meridian andconverging in another principal meridian. The modified beam is focusedto produce an astigmatic focal beam spot having an elongated shape andtwo separate focal points directing the astigmatic focal beam spot atthe substrate to obtain at least a partial cut in the substrate.

[0018] According to another aspect of the present invention, a method isprovided for scribing a semiconductor wafer using a laser. According tothis method, a laser beam is generated and an astigmatic focal beam spotis formed by modifying the laser beam such that the modified beam spothas two separate focal points and the astigmatic focal beam spot has anelongated shape. The astigmatic focal beam spot is directed at a surfaceof the semiconductor wafer. The astigmatic focal beam spot is appliedwith a set of parameters until one or more partial cuts are obtained inthe semiconductor wafer.

[0019] In accordance with another aspect of the present invention, amethod is provided for separating semiconductor wafers into dies.According to this method, a laser beam is generated and an astigmaticfocal beam spot is formed by modifying the laser beam such that themodified beam spot has two separate focal points and the astigmaticfocal beam spot has an elongated shape. The astigmatic beam spot isdirected at a surface of a semiconductor wafer. The variable astigmaticfocal beam spot is applied with a set of parameters until one or morepartial cuts are obtained in the semiconductor wafer. The semiconductorwafer is then separated into dies using the cuts.

[0020] According to yet another aspect of the present invention, a beamdelivery system comprises a beam expanding telescope for receiving a rawlaser beam from a laser and for producing an expanded beam. The beamdelivery system further comprises at least one variable anamorphic lenssystem comprising a cylindrical plano-concave lens and a cylindricalplano-convex lens for receiving the expanded beam and for producing amodified beam collimated in one principal meridian and converging inanother principal meridian. The beam delivery system further comprises abeam focusing lens for receiving the modified beam and focusing themodified beam such that the focused beam has two separate focal points.One of the focal points is shorter than a nominal focal length of thebeam focusing lens and the other of the focal points is formed generallyat the nominal focal length of the beam focusing lens.

[0021] According to yet another aspect of the present invention, amethod is provided for scribing a sapphire substrate having a layer ofGaN. This method comprises directing pulses of laser energy forming anastigmatic focal beam spot at a surface of the GaN on the sapphiresubstrate using a solid state laser and causing the pulses to impact thesapphire substrate in a scribe pattern to cut scribe lines in thesapphire substrate. The pulses couple laser energy into the GaN layer toinduce ablation of the sapphire substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] These and other features and advantages of the present inventionwill be better understood by reading the following detailed description,taken together with the drawings wherein:

[0023]FIG. 1 is a schematic diagram of a beam delivery system (BDS) withastigmatic focal point optics, according to one embodiment of thepresent invention.

[0024]FIG. 2 is a schematic diagram of the BDS shown in FIG. 1illustrating the sequential modification of the laser beam from thelaser to the target.

[0025]FIG. 3 is a cross-sectional view of a beam, illustrating theformation of two focal points separately in each principal meridian.

[0026]FIG. 4 is a cross-sectional view of a beam focusing lens in theBDS shown in FIG. 1, illustrating the ‘y component’ of the highlycompressed beam passing through the beam focusing lens.

[0027]FIG. 5 is a cross-sectional view of a beam focusing lens in theBDS shown in FIG. 1, illustrating the ‘x component’ of the highlycompressed beam passing through the beam focusing lens.

[0028]FIG. 6 is a cross-sectional view of the BDS shown in FIG. 1,illustrating the formation of two separated focal points in oneprincipal meridian.

[0029]FIG. 7 is a cross-sectional view of the BDS shown in FIG. 1,illustrating the formation of two separated focal points in the otherprincipal meridian.

[0030]FIGS. 8 and 9 are cross-sectional views of the BDS shown in FIG.1, illustrating the flexibility of adjusting processing parameters inthe BDS.

[0031]FIG. 10 is a photograph showing a top view of one example of asapphire-based LED wafer, scribed with the variable astigmatic focalbeam spot from the BDS using a 266 nm DPSS laser.

[0032]FIG. 11 is a photograph showing a cross-sectional view of oneexample of a sapphire-based LED wafer, scribed with the variableastigmatic focal beam spot.

[0033]FIG. 12 is a photograph showing a top view of one example of asilicon wafer, scribed with the variable astigmatic focal beam spot fromthe BDS using a 266 nm DPSS laser.

[0034]FIG. 13 is a photograph showing a top view of one example of a GaPwafer, scribed with the variable astigmatic focal beam spot from the BDSusing a 266 nm DPSS laser.

[0035]FIG. 14 is a photograph showing a top view of one example of amolybdenum film, scribed with the variable astigmatic focal beam spotfrom the BDS using a 266 nm DPSS laser.

[0036]FIG. 15 is a photograph showing one example of laser scribed lineswithout protective coating.

[0037]FIG. 16 is a photograph showing one example of laser scribed lineswith protective coating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] Overview of BDS

[0039] Referring to FIG. 1, one embodiment of an anamorphic beamdelivery system (BDS) 10 is described in detail. The anamorphic BDS 10generates an astigmatic focal beam spot that can be used to cut ormachine a substrate made of various types of materials. In one exemplaryapplication, the preferred embodiment of the BDS 10 improves theproductivity of LED die separation by forming a highly-resolvedadjustable astigmatic focal beam spot, which maximizes scribing speedand minimizes consumption of scribing-related real estate on a wafer.The BDS 10 can also be used in other scribing or cutting applications.

[0040] In the embodiment shown, a solid-state laser 12, preferably diodepumped, generates a laser beam in the UV range, preferably the thirdharmonic 355 nm or the fourth harmonic 266 nm. The raw laser beam isusually in TEM₀₀ mode with Gaussian distribution and is enlarged by abeam-expanding telescope (BET) 14. The exemplary embodiment of the BET14 is composed of the spherical plano-concave lens 16 and sphericalplano-convex lens 18. Magnification of the BET 14 is determined by thefocal lengths of each lens, generally described byM=(|f_(sx)|/|f_(sv)|), where M is magnification, f_(sx) is a focallength of the spherical plano-convex lens 18 and f_(sv) is a focallength of the spherical plano-concave lens 16. To effect collimated beamexpansion, the distance between the spherical plano-concave lens 16 andthe spherical plano-convex lens 18 is determined by a general equation,D_(c)=f_(sx)+f_(sv), where D_(c) is a collimation distance. Combinationsof f_(sx) and f_(sv) can be used to satisfy designed values of themagnification M and the collimation distance D_(c). The range of M canbe about 2× to 20×, and is preferably 2.5× in the exemplary BDS 10.Based on this preferred magnification of 2.5×, a combination off_(sx)=250 mm and f_(sv)=100 mm with D_(c)=150 mm is preferably used inthis BDS 10.

[0041] In the preferred embodiment, the expanded beam is reflected bythe 100% mirror 20 a and then directed to the beam shaping iris 22. Thebeam shaping iris 22 symmetrically crops out the low intensity edges ofthe beam in a Gaussian profile, leaving a high intensity portion passingthrough the iris 22. The beam is then directed to the center of avariable anamorphic lens system 24.

[0042] The exemplary variable anamorphic lens system 24 is composed of acylindrical plano-concave lens 26 and a cylindrical plano-convex lens28. The constituents of the variable anamorphic lens system 24preferably satisfy a condition, |f_(cx)|=|f_(cv)| where f_(cx) is afocal length of the cylindrical plano-convex lens 28 and f_(cv) is afocal length of the cylindrical plano-concave lens 26. In the variableanamorphic lens system 24, the incident beam is asymmetrically modifiedin one of the two principal meridians, which appears in the horizontaldirection in FIG. 1. In the anamorphic lens system 24, when D<D_(c),where D is a distance between a cylindrical plano-concave lens 26 and acylindrical plano-convex lens 28 and D_(c) is a collimation distance, aparallel incident beam is diverging after the anamorphic lens system 24.In contrast, when D>D_(c), a parallel incident beam is converging afterthe anamorphic lens system 24. In the preferred embodiment of theanamorphic lens system 24 shown in FIG. 1, the collimation distance isD_(c)=f_(cx)+f_(cv)=0, because |f_(cx)|=|f_(cv)| and f_(cx) has apositive value and f_(cv) a negative value and D≧D_(c), Accordingly,when D>0, the collimated incident beam is converging after theanamorphic lens system 24.

[0043] The degree of convergence or combined focal length (f_(as)) ofthe anamorphic system 24 is governed by the distance D, and it isgenerally expressed by the two lens principle:f_(as)=f_(cx)f_(cv)/(f_(cx)+f_(cv)−D). Namely, the larger the distanceD, the shorter the focal length f_(as). When the distance D increases,the degree of convergence increases in only one principal meridian ofthe collimated incident beam. One principal meridian of the incidentbeam loses its collimation and converges after the variable anamorphiclens system 24; however the other principal meridian is not affected andkeeps its beam collimation. Consequently, the size of the beam after thevariable anamorphic lens system 24 is changed in only one principalmeridian by adjusting the distance between the two lenses in theanamorphic system 24. Thus, the anamorphic BDS 10 deliberatelyintroduces astigmatism to produce focal points separated in twoprincipal meridians, i.e. vertical and horizontal. Although a series ofanamorphic lenses in different focal lengths or convergences ispreferred to provide a variable astigmatic beam spot, the variableanamorphic lens system can be replaced by a single anamorphic lens for afixed convergence.

[0044] After the variable anamorphic lens system 24, the beam isreflected by another 100% mirror 20 b, and then directed to the centerof a beam focusing lens 30. The exemplary beam focusing lens 30 is anaberration corrected spherical multi-element lens having a focal lengthrange between about +20 mm to +100 mm. In one embodiment of the BDS 10,an edge-contact doublet with +50 mm focal length is used. After the beamfocusing lens 30, one of the astigmatic focal points is sharply focusedon a substrate 32, such as a semiconductor wafer. In one preferredembodiment, the substrate 32 is translated by computer controlled x-ymotion stages 34 for scribing. In semiconductor scribing applicationswhere the semiconductor wafer contains square or rectangular dies, thesemiconductor wafer can be rotated 90 degrees by a rotary stage 36 forscribing in both the x direction and the y direction.

[0045] The preferred combination of the BET 14 and the multi-elementbeam focusing lens 30 yields a highly-resolved and adjustable astigmaticfocal beam spot with minimal aberration and a minimized beam waistdiameter. In general, a minimum beam waist diameter (w_(o)) of aGaussian beam can be expressed by: w_(o)=λf/πw_(i) where λ is awavelength of an incident laser beam, f is a focal length of a beamfocusing lens, π is the circular constant, and w_(i) is a diameter ofthe incident beam. In a given beam focusing lens 30, the minimum beamwaist diameter (w_(o)) or a size focused spot is inversely proportionalto the incident beam diameter (w_(i)). In the exemplary embodiment ofthe present invention, the BET 14 anamorphically increases the incidentbeam diameter (w_(i)) which is focused by the multi-element beamfocusing lens 30, resulting in a minimized beam waist diameter andyielding a highly-resolved focal beam spot. This provides a sharplyfocused scribing beam spot capable of providing about 5 μm or lessscribing kerf width on a semiconductor wafer. Consequently, theminimized scribing kerf width significantly reduces consumption of realestate on a wafer by scribing, which allows more dies on a wafer andimproves productivity.

[0046] The combination of the variable anamorphic lens system 24 and thehigh resolution beam focusing lens 30 results in two separate focalpoints in each principal meridian of the incident beam. The flexibilityof changing beam convergence from the variable anamorphic lens system 24provides an instant modification of a laser energy density on a targetsemiconductor wafer. Since the optimum laser energy density isdetermined by light absorption properties of the particular targetsemiconductor wafer, the variable anamorphic lens system 24 can providean instant adaptation to the optimum processing condition determined byvarious types of semiconductor wafers.

[0047] Although one exemplary embodiment of the anamorphic BDS 10 isshown and described, other embodiments are contemplated and within thescope of the present invention. In particular, the anamorphic BDS 10 canuse different components to create the astigmatic focal beam spot or theanamorphic BDS 10 can include additional components to provide furthermodification of the beam.

[0048] In one alternative embodiment, a bi-prism 38 or a set ofbi-prisms can be inserted between the anamorphic lens system 24 and theBET 14. The bi-prism equally divides the expanded and collimated beamfrom the BET 14, then crosses the two divided beams over to produce aninversion of half Gaussian profile. When a set of bi-prisms is used, thedistance between the two divided beams can be adjusted by changing thedistance between the set of bi-prisms. In other words, the bi-prism 38divides the Gaussian beam by half circles and inverts the two dividedhalf circles. A superimposition of these two circles createssuperimposition of the edges of Gaussian profiles in weak intensity.This inversion of a Gaussian profile and intensity redistributioncreates a homogeneous beam profile and eliminates certain drawbacks of aGaussian intensity profile.

[0049] In another embodiment, the BDS 10 can include an array ofanamorphic lens systems 24 used to create small segments of separatedastigmatic ‘beamlets’, similar to a dotted line. The astigmatic beamletsallow an effective escape of laser-induced plasma, which positivelyalters scribing results. The distance between the lenses in the array ofanamorphic lens systems controls the length of each segment of thebeamlets. The distance among the segments of the beamlets can becontrolled by introducing a cyclindrical plano-convex lens in front ofthe array of anamorphic lens systems.

[0050] Generation of Variable Astigmatic Focal Beam Spot

[0051] Referring to FIG. 2, one method of forming a variable astigmaticfocal beam spot is described in greater detail. The profile of raw beam50 from the laser generally has about 0.5 mm to 3 mm of diameter in aGaussian distribution. The raw beam 50 is expanded by the BET 14 and theexpanded beam 52 is about 2.5 times larger in diameter. The expandedbeam 52 is passed through the beam shaping iris 22 for edge cropping andthe expanded and edge-cropped beam 54 is directed to the center of theanamorphic lens system 24. The anamorphic lens system 24 modifies theexpanded and edge-cropped beam 54 in only one principle meridian,resulting in a slightly compressed beam shape 56. As the slightlycompressed laser beam 56 travels towards the beam focusing lens 30, thedegree of astigmatism is increased in the beam shape since the variableanamorphic lens system 24 makes the beam converge in only one principalmeridian. Subsequently, the highly compressed beam 57 passes through thebeam focusing lens 30 to form the astigmatic focal beam spot 58. Sincethe highly compressed beam 57 has converging beam characteristics in oneprincipal meridian and collimated beam characteristics in the other,focal points are formed separately in each principal meridian after thebeam focusing lens 30. Although this method of forming the astigmaticfocal beam spot 58 is described in the context of the exemplary BDS 10,this is not a limitation on the method.

[0052] The three-dimensional diagram in FIG. 3 illustrates in greaterdetail the formation of the two focal points separately in eachprincipal meridian when the highly compressed beam 57 passes through thebeam focusing lens (not shown). Since the highly compressed beam 57 inone principal meridian (hereinafter the ‘y component’) has convergingcharacteristics, the y component exhibits the short distance focal point60. In contrast, since the other meridian (hereinafter the ‘xcomponent’) has collimating beam characteristics, the x componentexhibits the long distance focal point 62. Combination of the x and ycomponents results in the astigmatic beam spot 58.

[0053]FIG. 4 shows the y component of the highly compressed beam 57,which passes through the beam focusing lens 30 and results in the focalpoint 60. After the focal point 60, the beam diverges and creates theastigmatic side of the beam spot 58.

[0054]FIG. 5 shows the x component of the highly compressed beam 57,which passes through the beam focusing lens 30 and results in the focalpoint 62. The collimated x component of the highly compressed beam 57 issharply focused at the focal point 60, which creates the sharply focusedside of the astigmatic beam spot 58.

[0055]FIGS. 6 and 7 illustrate further the formation of two separatedfocal points 60, 62 in each principal meridian. The schematic beamtracings in FIGS. 6 and 7 include two-dimensional layouts of the BDS 10shown in FIG. 1 excluding the 100% mirrors 20 a, 20 b and the beamshaping iris 22 for simplicity. In FIG. 6, the raw beam from thesolid-state laser 12 is expanded by the BET 14 and then collimated. Thevariable anamorphic lens system 24 modifies the collimated beam in thisprinciple meridian, resulting in convergence of the beam. The convergingbeam is focused by the beam focusing lens 30. Due to its convergencefrom the variable anamorphic lens system 24, the beam forms the focalpoint 60, shorter than the nominal focal length of the beam focusinglens 30. The beam tracing in FIG. 6 is analogous to the view of the ycomponent in FIG. 4.

[0056] In contrast, in FIG. 7, the expanded and collimated beam from BET14 is not affected by the variable anamorphic lens system 24 in thisprincipal meridian. The collimation of the beam can be maintained inthis meridian after the variable anamorphic lens system 24. Afterpassing though the beam focusing lens 30, the collimated beam is focusedat the focal point 62, which is formed at a nominal focal length of thebeam focusing lens 30. The beam tracing in FIG. 7 is analogous to theview of the x component in FIG. 5. In FIG. 7, the BET 14 increases theincident beam diameter, which is focused by the multi-element beamfocusing lens 30, resulting in minimized a beam waist diameter andyielding a highly-resolved focal beam spot. As a result, the targetsubstrate 32 (e.g., a semiconductor wafer) receives a wide and defocusedastigmatic beam in one principal meridian and a narrow and sharplyfocused beam in the other principal meridian.

[0057] As illustrated in FIG. 3, the combination of these two separatedfocal points 60, 62 generates an astigmatic beam spot having one sidewith a defocused and compressed circumference and the other side with asharply focused and short circumference.

[0058] Scribing Applications Using an Astigmatic Focal Beam Spot

[0059] To scribe a substrate, the astigmatic focal beam spot is directedat the substrate and applied with a set of parameters (e.g., wavelength,energy density, pulse repetition rate, beam size) depending upon thematerial being scribed. According to one method, the astigmatic focalbeam spot can be used for scribing semiconductor wafers, for example, inwafer separation or dicing applications. In this method, the wafer canbe moved or translated in at least one cutting direction under thefocused laser beam to create one or more laser scribing cuts. To cutdies from a semiconductor wafer, a plurality of scribing cuts can becreated by moving the wafer in an x direction and then by moving thewafer in a y direction after rotating the wafer 90 degrees. Whenscribing in the x and y directions, the astigmatic beam spot isgenerally insensitive to polarization factors because the wafer isrotated to provide the cuts in the x and y directions. After thescribing cuts are made, the semiconductor wafer can be separated alongthe scribing cuts to form the dies using techniques known to thoseskilled in the art.

[0060] The astigmatic beam spot provides an advantage in scribingapplications by enabling faster scribing speeds. The scribing speed canbe denoted by S=(l_(b)·r_(p))/n_(d), where S is the scribing speed(mm/sec), l_(b) is the length of the focused scribing beam (mm), r_(p)is pulse repetition rate (pulse/sec) and n_(d) is the number of pulsesrequired to achieve optimum scribing cut depth. The pulse repetitionrate r_(p) depends on the type of laser that is used. Solid state laserswith a few pulses per second to over 10⁵ pulses per second arecommercially available. The number of pulses n_(d) is a materialprocessing parameter, which is determined by material properties of thetarget wafer and a desired cut depth. Given the pulse repetition rater_(p) and the number of pulses n_(d), the beam length l_(b) is acontrolling factor to determine the speed of the cut. The focusedastigmatic beam spot formed according to the method described aboveincreases the beam length l_(b) resulting in higher scribing speeds.

[0061] The preferred BDS 14 also provides greater flexibility to adjustprocessing parameters for achieving an optimum condition. In lasermaterial processing, for example, processing parameters shouldpreferably be adjusted for optimum conditions based on materialproperties of a target. The overflow of laser energy density can resultin detrimental thermal damage to the target, and the lack of laserenergy density can cause improper ablation or other undesired results.FIGS. 8 and 9 show the flexibility of adjusting processing parameters ofthe BDS in this invention.

[0062] In FIG. 8, the lenses 26, 28 of the variable anamorphic lenssystem 24 are placed close together, which results in low convergence ofthe collimated incident beam. This low convergence forms the focal point60 at a relatively further distance from the beam focusing lens 30.Consequently, the length of the beam spot 58 is relatively shorter.

[0063] In contrast, in FIG. 9, the lenses 26, 28 of the variableanamorphic lens system 24 are placed further apart, which results inhigh convergence of the collimated incident beam. This increasedconvergence introduces astigmatism and forms the focal point 60 at arelatively shorter distance from the beam focusing lens 30.Consequently, the length of the beam spot 58 is relatively longer.

[0064] In one scribing example, the astigmatic focal beam spot can beused to scribe a sapphire substrate used for blue LEDs. Optimumprocessing of a sapphire substrate for blue LEDs generally requires anenergy density of about 10 J/cm². Since blue LED wafers are generallydesigned to have about a 50 μm gap among the individual die forseparation, the optimum laser beam size is preferably less than about 20μm for laser scribing. When a currently-available commercial laser with3 Watts on target output at 50 kHz pulse repetition is used, theconventional beam focusing at a 15 μm diameter results in laser energydensity of 34 J/cm². In a system with conventional beam spot focusing,the energy density on target has to be adjusted by reducing the poweroutput of the laser for optimum processing to avoid an overflow. Thus,the laser power output cannot be fully utilized to maximize the scribingspeed or productivity.

[0065] In contrast, the preferred embodiment of the BDS 10 can adjustthe size of the compressed beam spot to maintain the optimum laserenergy density for 10 J/cm² without reducing the power output from thelaser. The size of the astigmatic beam can be adjusted to have about 150μm in the astigmatic axis and about 5 μm in the focused axis. Since theastigmatic axis is lined up in the scribing translation direction, thisincrease in beam length proportionally increases the scribing speed asdiscussed above. In this example, the astigmatic beam spot can provideprocessing speeds that are about 10 times faster than that ofconventional beam focusing.

[0066] In another scribing example, the astigmatic focal beam spot canbe used to scribe a sapphire substrate by coupling with one or more GaNlayers on the sapphire substrate (e.g., about 4˜7 μm over the sapphiresubstrate) instead of coupling directly with sapphire. The lower bandgapof GaN provides more efficient coupling with the incident laser beam,requiring only about 5 J/cm² for the laser energy density. Once thelaser beam couples with GaN, the ablation through the sapphire substrateis much easier than direct coupling with the sapphire. Accordingly, thesize of the astigmatic beam can be adjusted to have about 300 μm in theastigmatic axis and about 5 μm in the focused axis. Thus, the processingspeed can be 20 times faster than the conventional far field imaging orspot focusing techniques.

[0067] The minimized spot size in the focused axis also significantlyreduces the scribing kerf width, which subsequently reduces consumptionof a wafer real estate. Furthermore, by reducing total removed materialvolume, the narrow scribing cuts reduce collateral material damage andablation-generated debris. FIG. 10 shows an example of a sapphire basedLED wafer, scribed with the astigmatic focal beam spot from the BDS 10using a 266 nm DPSS laser with on target power of about 1.8 Watt at 50kHz. The size of the astigmatic beam was adjusted to have about 180 μmin the astigmatic axis and about 5 μm in the focused axis. Viewed fromthe top, FIG. 10 shows a cut width of about 5 μm. Based on 30 μm deepscribing, the BDS 10 is capable of scribing speeds of greater than 50mm/sec. The laser cut forms a sharp V-shaped groove, as shown in FIG.11, which facilitates well controlled fracturing after the scribing. Thevariable astigmatic focal beam spot from the adjustable BDS 10 utilizesthe maximum power output from the laser, which directly increases theprocessing speeds. Thus, front side scribing can be used to decrease thestreet width and increase fracture yield, thereby increasing usable dieper wafer.

[0068] The astigmatic focal beam spot can also be used advantageously toscribe other types of semiconductor wafers. The astigmatic focal beamspot readily adjusts its laser energy density for an optimum value,based on the target material absorption properties, such as bandgapenergy and surface roughness. FIG. 12 shows an example of a siliconwafer, scribed with the astigmatic focal beam spot from the BDS 10 usinga 266 nm DPSS laser with on target power of about 1.8 Watt at 50 kHz.The size of the astigmatic beam was adjusted to have about 170 μm in theastigmatic axis and about 5 μm in the focused axis. This resulted in 75μm deep scribing with a speed at about 40 mm/sec.

[0069] A comparable result is shown in FIG. 13 for a GaP wafer with thesame laser and on target power. The size of the astigmatic beam wasadjusted to have about 300 μm in the astigmatic axis and 5 μm in thefocused axis. This resulted in 65 μm deep scribing with a speed at about100 mm/sec. Similar results were found in other compound semiconductorwafers such as GaAs and Ge. Other substrates that can be scribedinclude, but are not limited to, InP, Alumina, glass and polymers.

[0070] The astigmatic focal beam spot can also be used advantageously toscribe or machine metal films. Due to high thermal conductivity, lasercutting of metal films using conventional techniques has shown extensiveheat affected zones along the wake of the laser cut. With theapplication of the astigmatic focal beam spot, the 5 μm beam width inthe focused axis significantly reduces a laser cutting kerf width, whichsubsequently reduces heat affected zones, collateral material damage andablation-generated debris. As an example, FIG. 14 shows narrow andshapely resolved cut lines on molybdenum. The size of the astigmaticbeam was adjusted to have about 200 μm in the astigmatic axis and about5 μm in the focused axis. This resulted in 50 μm deep scribing with aspeed at about 20 mm/sec, using 266 nm DPSS laser with on target powerof about 2.5 Watt at 25 kHz. Other types of metal can also be cut.

[0071] Although the examples show lines scribed in a substrate, theastigmatic focal beam spot can also be used to scribe other shapes or toperform other types of machining or cutting applications. Operatingparameters other than those given in the above examples are alsocontemplated for scribing LED wafers. For example, a 355 nm DPSS lasercan also be used for LED scribing applications, although the 266 nm DPSSlaser is preferable to minimize the thermal damage that can cause lowerlight output of the LED.

[0072] According to another scribing method, surface protection can beprovided on the substrate by using a water soluble protective coating.The preferred composition of the protective coating comprises at leastone surfactant in a water-soluble liquid glycerin and can be any kind ofgeneric liquid detergent that satisfies this compositional requirement.The surfactant in the liquid glycerin forms a thin protective layer dueto its high wetability. After the thin film layer is dried off, theglycerin effectively endures heat from the laser induced plasma, whilepreventing laser generated debris from adhering on the surface. The thinfilm of liquid detergent is easily removed by cleaning with pressurizedwater. FIG. 14 illustrates the laser scribing on a LED wafer without thesurface protection, showing a significant amount of debris accumulatedalong the laser cut. In contrast, FIG. 15 illustrates the laser scribingon a LED with protective coating using a liquid detergent, whichprevented the laser induced debris on the LED surface.

[0073] Accordingly, the preferred embodiment of the present inventionprovides advantages over conventional systems using patterned laserprojection and conventional systems using far field imaging. Unlikesimple far field imaging, the present invention provides greaterflexibility for modifying the laser beam by using the anamorphic BDS toproduce the astigmatic focal beam spot. Unlike conventional patternedlaser projection, the anamorphic BDS delivers substantially the entirebeam from a laser resonator to a target, thus maintaining very high beamutilization. The formation of the astigmatic focal beam spot also allowsthe laser beam to have excellent characteristics in both the optimumintensity and the beam waist diameter. In particular, the preferredembodiment of the variable anamorphic lens system enables an adjustableuniplanar compression of a laser beam, which results in a variable focalbeam spot for prompt adjustments of the optimum laser intensity. Byproper modification of beam spot and by maximized utilization of a rawbeam, the formation of the astigmatic focal beam spot results innumerous advantages on separation of various semiconductor wafers,including fast scribing speeds, narrow scribing kerf width, reducedlaser debris, and reduced collateral damage.

[0074] While the principles of the invention have been described herein,it is to be understood by those skilled in the art tat this descriptionis made only by way of example and not as a limitation as to the scopeof the invention. Other embodiments are contemplated within the scope ofthe present invention in addition to the exemplary embodiments shown anddescribed herein. Modifications and substitutions by one of ordinaryskill in the art are considered to be within the scope of the presentinvention, which is not to be limited except by the following claims.

The invention claimed is:
 1. A method for forming a variable astigmaticfocal beam spot to cut a substrate, said method comprising the steps of:generating a raw laser beam; expanding said raw laser beam; modifyingsaid expanded beam such that said modified beam is collimated in oneprincipal meridian and converging in another principal meridian;focusing said modified beam having two separate focal points to producean astigmatic focal beam spot having an elongated shape; and directingsaid astigmatic focal beam spot at said substrate to obtain at least apartial cut in said substrate.
 2. The method of claim 1 wherein said rawbeam is generated using a solid state laser.
 3. The method of claim 2wherein said raw beam is generated in a UV range less than about 400 nm.4. The method of claim 3 wherein said raw beam is generated with a pulseduration less than about 40 ns.
 5. The method of claim 1 wherein thestep of expanding said raw beam includes passing said raw beam through abeam expanding telescope.
 6. The method of claim 1 wherein the step ofmodifying said expanded beam includes passing said expanded beam throughan anamorphic lens system comprising a cylindrical plano-concave lensand a cylindrical plano-convex lens.
 7. The method of claim 1 furthercomprising the step of varying the convergence of said modified beam. 8.The method of claim 1 wherein the step of modifying said expanded beamincludes passing said expanded beam through a single anamorphic lens toprovide a fixed convergence.
 9. The method of claim 1 further comprisingthe step of symmetrically cropping low intensity edges of said expandedbeam.
 10. The method of claim 1 wherein the step of focusing saidmodified beam comprises passing said modified beam through a beamfocusing lens, wherein said focused beam has two separate focal points,wherein one of said focal points is shorter than a nominal focal lengthof said beam focusing lens and the other of said focal points is formedgenerally at said nominal focal length of said beam focusing lens. 11.The method of claim 1 wherein said substrate includes sapphire.
 12. Themethod of claim 11 wherein said substrate includes a GaN layer on saidsapphire, and wherein said astigmatic focal beam spot is directed at asurface of said GaN layer such that laser energy is coupled into saidGaN layer to cause ablation of said sapphire.
 13. The method of claim 1wherein said substrate is part of a semiconductor wafer including adevice layer on said substrate.
 14. The method of claim 1 wherein saidsubstrate is made of a material selected from the group consisting ofmetal, GaAs, silicon, GaP, InP, Ge, alumina, glass and polymers.
 15. Themethod of claim 1 wherein said substrate includes a metal film made of ametal selected from the group consisting of molybdenum and copper. 16.The method of claim 1 wherein said astigmatic focal beam spot has awidth of less than about 20 μm.
 17. The method of claim 16 wherein saidastigmatic focal beam spot has a width of about 5 μm.
 18. The method ofclaim 1 further comprising the step of moving said substrate in acutting direction along a length of said astigmatic focal beam spot. 19.The method of claim 1 wherein the step of modifying said expanded beamincludes creating a plurality of separated astigmatic beamlets.
 20. Themethod of claim 19 wherein the step of modifying said expanded beamincludes controlling at least one of a length of said beamlets and adistance between said beamlets.
 21. The method of claim 1 furthercomprising the step of applying a water soluble protective coating tosaid substrate before directing said astigmatic focal beam spot at saidsubstrate, said protective coating including at least one surfactant ina water-soluble liquid glycerin.
 22. A method for scribing asemiconductor wafer using a laser, said method comprising the steps of:generating a laser beam; forming at least one astigmatic focal beam spotby modifying said laser beam such that said modified beam has twoseparate focal points and said astigmatic focal beam spot has anelongated shape; and directing said astigmatic focal beam spot at asurface of said semiconductor wafer, wherein said astigmatic focal beamspot is applied with a set of parameters until at least a partial cut insaid semiconductor wafer is obtained.
 23. The method of claim 22 furthercomprising: moving said semiconductor wafer in a cutting direction alonga length of said astigmatic focal beam spot to make at least one cut inan x direction on said semiconductor wafer; rotating said semiconductorwafer about 90 degrees; and moving said semiconductor wafer in a cuttingdirection along a length of said astigmatic focal beam spot to make atleast one cut in a y direction on said semiconductor wafer.
 24. Themethod of claim 22 wherein the step of forming said astigmatic beam spotuses an anamorphic lens system to receive an expanded, collimated beamand to produce a modified beam that is collimated in one principalmeridian and converging in another principal meridian.
 25. The method ofclaim 22 wherein the step of forming said astigmatic beam spot uses abeam focusing lens to focus said beam spot such that one of said focalpoints is shorter than a nominal focal length of said beam focusing lensand the other of said focal points is formed generally at said nominalfocal length of said beam focusing lens.
 26. The method of claim 22further comprising the step of varying said astigmatic focal beam spot.27. The method of claim 24 wherein said anamorphic lens system comprisesa cylindrical plano-concave lens and a cylindrical plano-convex lens.28. The method of claim 27 further comprising the step of varying saidastigmatic focal beam spot by varying a spacing between said cylindricalplano-concave lens and said cylindrical plano-convex lens.
 29. Themethod of claim 22 wherein the step of forming said astigmatic focalbeam spot comprises the steps of: generating a raw laser beam; expandingsaid raw laser beam; modifying said expanded beam such that saidmodified beam is collimated in one principal meridian and converging inanother principal meridian; and focusing said modified beam to producesaid astigmatic focal beam spot.
 30. The method of claim 22 wherein thestep of forming said at least one astigmatic focal beam spot includesforming small segments of separated astigmatic beamlets.
 31. The methodof claim 22 further comprising the step of translating saidsemiconductor wafer in a cutting direction along a length of saidastigmatic focal beam spot to make a plurality of cuts in saidsemiconductor wafer.
 32. The method of claim 22 wherein saidsemiconductor wafer includes a sapphire substrate.
 33. The method ofclaim 32 wherein said semiconductor wafer includes a GaN layer on saidsapphire substrate, and wherein said astigmatic focal beam spot isdirected at said GaN layer such that laser energy is coupled into saidGaN layer to cause ablation of said sapphire.
 34. A method forseparating semiconductor wafers into dies, said method comprising thesteps of: generating a laser beam; forming at least one astigmatic focalbeam spot by modifying said laser beam such that said modified beam hastwo separate focal points and said astigmatic focal beam spot has anelongated shape; directing said astigmatic focal beam spot at a surfaceof a semiconductor wafer, wherein said astigmatic focal beam spot isapplied with a set of parameters until at least a partial cut in saidsemiconductor wafer is obtained; and separating said semiconductor waferinto dies using said at least a partial cut.
 35. A beam delivery systemcomprising: a beam expanding telescope for receiving a raw laser beamfrom a laser and for producing an expanded beam; at least one variableanamorphic lens system comprising a cylindrical plano-concave lens and acylindrical plano-convex lens for receiving said expanded beam and forproducing a modified beam collimated in one principal meridian andconverging in another principal meridian; and a beam focusing lens forreceiving said modified beam and focusing said modified beam such thatsaid focused beam has two separate focal points, wherein one of saidfocal points is shorter than a nominal focal length of said beamfocusing lens and the other of said focal points is formed generally atsaid nominal focal length of said beam focusing lens.
 36. The beamdelivery system of claim 35 further comprising a beam shaping irisbetween said beam expanding telescope and said variable anamorphic lenssystem for cropping out low intensity edges of said expanded beam. 37.The beam delivery system of claim 35 further comprising at least onebi-prism between said beam expanding telescope and said variableanamorphic lens system for dividing said expanded beam.
 38. The beamdelivery system of claim 35 further comprising a set of bi-prismsbetween said beam expanding telescope and said variable anamorphic lenssystem for dividing said expanded beam and controlling separation ofsaid divided beams.
 39. The beam delivery system of claim 35 furthercomprising an array of variable anamorphic lens systems for creating aplurality of separated astigmatic beamlets.
 40. A method for scribing asapphire substrate having a layer of GaN, said method comprising thesteps of: directing pulses of laser energy forming an astigmatic focalbeam spot at a surface of said GaN on said sapphire substrate using asolid state laser, wherein said pulses couple laser energy into said GaNlayer to induce ablation of said sapphire substrate; and causing saidpulses to impact said sapphire substrate in a scribe pattern to cutscribe lines in said sapphire substrate.